1. Field of the Invention
The invention relates to the field of seismic data processing. More precisely, it relates to the inversion of seismic data.
2. Description of the Related Art
In general, for seismic exploration, a plurality of sources and receivers are distributed at ground level and at a distance from each other. The seismic sources are activated to generate seismic waves that are propagated in the subsurface. These seismic waves undergo deviations during their propagation. They are refracted, reflected and diffracted at the interfaces of the subsurface. Certain waves propagated in the subsurface are detected by seismic receivers and recorded in real time in the form of signals (called traces). The recorded traces can be processed to obtain an image of the subterranean geological structures.
During processing, the summing step (or stacking) consists of adding together the traces corresponding to seismic waves that are reflected at a same point in the subsurface. This step makes it possible to increase the signal-to-noise ratio and the primary-reflections-to-multiple-reflections ratio in the seismic data processed.
Starting from the assumption of a subsurface that is horizontally stratified without lateral variation of propagation velocities, it can be shown that the traces having the property of illuminating the same point of the subsurface for variable source-receiver distances (or offset) are those having the same mid-point in common between source and receiver.
However, the waves reflected in the subsurface are recorded at variable times according to the offset. Before adding the traces, it is therefore necessary to correct these traces to reduce them to a common reference, the zero offset trace. This correction is carried out during a step called NMO (Normal Move Out) correction.
The NMO step requires prior knowledge of a model for propagation velocities of seismic waves in the subsurface.
For example, the Dix model is based on the assumption that the subsurface is formed of horizontal layers or strata in which each layer is isotropic and has an associated given propagation velocity (interval velocity). The NMO correction relies on the model thus defined to correct the arrival time of a reflection recorded with a given offset x by bringing it to the theoretical time τ0 at which it would have been recorded with a zero offset x=0.
Given that the velocities are not known a priori, the correction step is carried out on the seismic traces by sweeping a range of velocities. Next, only the velocity optimising the semblance of the traces as a whole is retained.
Thus it is possible, in the most favourable cases, to deduce an estimate of the interval velocities between the highest energy events reflected.
In general the estimation of interval velocities does not take into account the anisotropy of the subsurface, that is to say the variation of velocity in the layers as a function of the propagation direction.
Moreover, neither does the estimate of interval velocities take into account the variation in reflectivity of the subsurface interfaces as a function of the angle of incidence of the wave.